Marek's Disease (MD)

What is the definition of Marek's diease?

Marek's disease is a herpesvirus infection, primarily of young chickens, that causes lymphoma of T lymphocytes. Tumors may occur in the nerves, ovaries, testes, viscera, eye, muscle, and skin. MD is ubiquitous throughout the world. The leg paralysis was often referred to as range paralysis.

What causes Marek's disease? What are the 3 serotypes of the virus?

herpes virus (dsDNA) serotypes 1) oncovirus (tumor-causing) 2) non-oncogenic 3) herpes virus turkey (HVT)

Can this virus live outside host cells for very long?

no ● The virus is intranuclear (cell-associated) and normally cannot live outside the host cell being protected from the environment by host epithelium.

What is the treatment?

no effective treatment for chickens emphasis on prevention

What are the 3 phases of virus-cell interactions that allows viral replication to occur?

1) productive - Occurs mainly in nonlymphocytic cells with replication of viral DNA, antigen synthesized and viral particles are sometimes produced. Full productive infection occurs in feather follicle epithelium that develops large numbers of enveloped and fully infectious viruses 2) latent - Occurs in lymphocytes, predominantly T cells, but occasionally B cells. The viral genome is not expressed and no virus or tumor-associated antigen is produced. 3) transformation - Occurs in only T lymphocytes and is caused by only the virulent serotype 1 Marek's disease viruses. This transformation in T lymphocytes results in the formation of neoplastic lymphocytes with viral antigen expressed. MD tumor-associated antigen is expressed on the cell surface.

How is it diagnosed?

● A presumptive diagnosis is made based on the presence of tumors and the observed paralysis; however, gross necropsy and histopathology are important to make a definitive diagnosis and to differentiate Marek's disease from other forms of paralysis. ● PCR for MD viral nucleic acid can be conducted on peripheral blood white cell buffy coats or lymphoid tumors as confirmation; this is not readily available for backyard chickens because of expense and lack of laboratory testing.

What are the gross lesions?

● Birds can have one or both neural and visceral lesions. ● Affected nerves (sciatic, brachial and pelvic plexus) are characterized by swelling, loss of cross-striations, and are gray or yellow. ● Lymphoid tumors may be found in the gonads, ● heart, liver, lung, kidney, spleen, bursa, intestines, muscle, and skin.

What are the clinical signs?

● Clinical signs and gross lesions generally do not appear until 4 weeks of age or later. ● In acute outbreaks, birds become severely depressed and uncoordinated followed by unilateral or bilateral paralysis of legs and wings. ● Many birds become dehydrated, emaciated, and eventually die. ● The extremities affected include the legs, wings, and neck. ● In an infected flock, mortality gradually builds and generally persists for 4-10 weeks. ● Ocular Marek's disease is characterized by decreased pupil size and irregular diameter ("grey eye"). ● Immunosuppression can be a long-term effect.

How is Marek's disease transmitted? How long can shedding occur?

● Infectious virus is produced only in feather follicle epithelium and spreads by direct or indirect contact between birds. ● The infectious virus contaminates the premises through infected molted feathers and dander. ● Birds become infected when they inhale dust containing the virus. Many apparently normal birds are carriers that can transmit the infection. ● Some birds have been found to shed the virus from skin for as long as eighteen months.

How is it prevented?

● Vaccination against MD is effective in controlling the disease. ● Marek's disease vaccine is usually administered on day 1 or injected into the embryo three days prior to hatch (in ovo). ● There are three types of vaccines commercially available: the HVT serotype 3, natural occurring avirulent isolates of serotype 2, and non-oncogenic strains of serotype 1 (Rispens). ● MD vaccines often achieve over 90% protection under commercial conditions. At least 1000-1500 plaque-forming units of live virus are needed per dose. ● Chicks are normally vaccinated at the hatchery because they are exposed to the field virus at an early age. ● Cell-associated vaccines require extreme care in handling and application. ● Condemnations due to Marek's Disease can often be traced to errors in vaccination